Active implantable medical devices are known to be sensitive to the detection of signals having a non-cardiac origin, for example, due to issues with a lead, electromagnetic interference, detection of myopotentials. These phenomena are hereinafter individually and collectively referred to under the generic term “noise”. These noise phenomena are likely to generate artifacts that can lead to serious consequences, if not appropriately interpreted by a medical device, for example, by wrongly inhibiting bradycardia stimulations or resynchronization therapies, or conversely, by allowing delivery of inappropriate defibrillation shocks.
In particular, it was observed by Schulte B, et al. in Inappropriate Arrhythmia Detection in Implantable Defibrillator Therapy Due to Oversensing of diaphragmatic Myopotentials, J Interv Card Electrophysiol, 2001. 5 (4): p. 487-93, that among a group of 384 patients implanted with a defibrillator, 139 episodes of arrhythmia were incorrectly identified as ventricular fibrillation due to the detection of myopotentials in 33 patients (8.6%), and 32 inappropriate shocks were delivered to 11 patients (2.9%). Similarly, Occhetta E, et al. in Inappropriate Implantable Cardioverter-Defibrillator Discharges Unrelated to Supraventricular Tachyarrhythmias, Europace, 2006. 8 (10): p. 863-9, showed that, overall, 4% of the patients implanted with a defibrillator receive at least one inappropriate shock due to “over-detection” of extra-cardiac signals that are clinically irrelevant to supraventricular arrhythmias.
The application of a defibrillation shock to a conscious patient is generally painful and agonizing, because the applied energies are well beyond a patient's threshold of pain. In addition, the application of a defibrillation shock presents side effects on the heart rate (i.e., there is a risk of developing a secondary disorder), on the functional integrity of the myocardium, and generally on the patient's physiological balance. It is, therefore, important to deliver such shocks only when appropriate, and only when a less painful alternative therapy, such as an appropriate stimulation of the atrium, is not feasible.
Commonly used methods to filter noise signals in ventricular signals include filtering the collected signals and automatically controlling various detection parameters such as a blanking period, a sensitivity value, etc. to detect over-detections, and prohibit any inappropriate action.
U.S. Pat. No. 6,584,351 B1 proposes a method that attempts a priori to eliminate noise signals before ventricular sensing. To this end, an additional electrode, externally located from the heart, detects signals having a non-cardiac origin, which are considered as noise. These noise signals are then subtracted from the detected signals by the intracardiac electrodes. For this method to be effective, however, it requires implantation of an external electrode outside the heart, for example, on the housing of the medical device—this implicates a redesign of the mechanical and electrical structure of the medical device.
Other methods have been proposed to operate a “post-filtering” of noise signals contained in the extra-cardiac ventricular signal after they are collected. U.S. Pat. No. 7,567,835 B2 proposes to compare a ventricular marker chain produced by the defibrillator from a bipolar (nearfield) signal with an R marker chain obtained by analyzing a unipolar (farfield) signal that is simultaneously collected. The markers detected by the defibrillator from the nearfield signal, but are not detected on the farfield signal, are considered as noise. The origin of these noise signals are determined by analyzing the number and the pattern of the markers in the chains on the two channels. This technique has a disadvantage that, if an extra-cardiac signal from outside the heart is present on both channels (nearfield and farfield), it is not considered as noise, even if this phenomenon is clearly identifiable by a visual examination of the traces.
U.S. Pat. No. 7,215,993 B2, proposes to calculate a weighted average and normalized amplitude from: the amplitude of the analyzed beat, the amplitude of the preceding beat, and the amplitude of the following beat. If the average exceeds a predetermined threshold, the analyzed beat is considered valid, otherwise it is regarded as noise. However, this method is sensitive to various rhythm abnormalities occurring erratically, which may lead to both false positive and false negative results.